1. Field of the Invention
The present invention relates to a magnetic recording medium for use in a hard disk drive using the magnetic recording technique, and to a magnetic recording/reproducing apparatus using the magnetic recording medium.
2. Description of the Related Art
A magnetic storage device (hard disk drive;HDD) mainly used in computers to record and reproduce information is recently gradually extending its applications because of its large capacity, inexpensiveness, high data access speed, data holding reliability, and the like. The HDD is now used in various fields such as household video decks, audio apparatuses, and car navigation systems. As the range of use of the HDD widens, demands for increasing the storage capacity or density of the HDD are also increasing. In recent years, high-density HDDs are being more and more extensively developed.
The longitudinal magnetic recording system is used in magnetic recording/reproducing apparatuses presently put on the market. In a magnetic recording layer used, magnetic grains for recording information have an easy axis of magnetization parallel to the substrate. The easy axis of magnetization is an axis in the direction of which magnetization easily points. In the case of a Co-based alloy, the c axis of the hexagonal closest packed structure of Co is the easy axis of magnetization. As the recording density increases, recording bits of a magnetic layer become small. If the recording bits are too small, information in these recording bits may be thermally erased by a so-called thermal decay effect, in other words, the recording/reproduction characteristics are worsened. Additionally, in a longitudinal magnetic recording medium, as the recording density increases, noise generated from the medium often increases due to the influence of an antimagnetic field generated in the boundary between the recording bits.
In contrast, in a so-called perpendicular magnetic recording system in which the easy axis of magnetization in the magnetic recording layer is aligned substantially perpendicularly to the substrate. The influence of an antimagnetic field between recording bits is small even when the recording density increases. The magnetization is magnetostatically stable even at high density. Therefore, this perpendicular magnetic recording system is recently very noted as a technique, which replaces the longitudinal recording system. The perpendicular magnetic recording medium is generally formed by a magnetic recording layer made up a hard magnetic material, an orientation control undercoating layer for orienting a specific crystal axis of the magnetic recording layer, and a protective layer for protecting the surface of the magnetic recording layer. In this perpendicular magnetic recording medium, a soft magnetic backing layer for concentrating a magnetic flux generated by a magnetic head used in recording to the magnetic recording layer is often formed between the magnetic recording layer and the substrate.
Even in the perpendicular magnetic recording medium, to increase the recording density, it is being desired to reduce noise while the thermal stability is maintained. Various methods can be used to obtain a fine structure in order to realize low noise. Generally, a method of decreasing the grain size of magnetic grains in the magnetic recording layer is used. In the case of a CoCr-based magnetic recording layer presently extensively used in both the longitudinal and perpendicular magnetic recording media, it has been attempted to decrease the grain size of magnetic grains by adding Ta or B to the magnetic recording layer or heating the layer at an appropriate temperature, thereby segregating nonmagnetic Cr in its grain boundary. However, the degree of this Cr segregation in the perpendicular magnetic recording medium is smaller than that in the longitudinal magnetic recording medium. Therefore, separation between the magnetic grains is insufficient, so the magnetic interaction between the grains remains relatively large. This poses the problem that transition noise between recording bits cannot be well reduced.
As another method of decreasing the size of magnetic grains, it is possible to decrease the grain size of an orientation control undercoating layer which is formed below a magnetic layer in order to control the orientation of the crystal axis and grain size of the magnetic layer. For example, Jpn. Pat. Appln. KOKAI Publication No. 2003-36525 discloses a method which uses an undercoating layer having a so-called granular structure made up of crystal grains and a grain boundary region separating the crystal grains by using an oxide or nitride as an additive, thereby downsizing and separating the crystal forming the undercoating layer. Also, Jpn. Pat. Appln. KOKAI Publication No. 2002-25030 discloses a method of giving both a magnetic recording and undercoating layer a granular structure by using, e.g., titanium oxide or silicon oxide as an additive. However, if the amount of additive is increased to further increase the recording density, the magnetic recording layer readily degrades its orientation or magnetic characteristics. In addition, in a film having a granular structure, the orientation of crystal grains is often inferior even though downsizing of these crystal grains is achieved. Accordingly, because the downsizing of the magnetic recording layer formed on the undercoating layer often makes the crystallinity worse, it is impossible to well bring out the characteristics of the magnetic recording medium by the worse crystallinity of the magnetic recording layer. As described above, to obtain a perpendicular magnetic recording medium capable of high-density recording, further improvements of the structure, particularly the undercoating layer are being required.